In recent years reverse osmosis has become attractive for utilization in the purification of sea water, saline water, for the recovery of valuable materials from waste liquids, etc. Various semipermeable membranes are now being used in commercial osmosis treatment of aqueous solutions either for water purification or for concentration of a liquid solution, or both. Such semipermeable membranes include the early "Loeb" type membranes made of cellulose acetate by processes as described in U.S. Pat. Nos. 3,133,132 and 3,133,137. The Loeb-type membranes are asymmetric types and are characterized by a very thin, dense surface layer or skin that is supported upon an integrally attached, much thicker supporting layer. However, the cellulose acetate Loeb-type membranes are restricted in use and in processability, largely because the membranes must be kept wet at all times, that is, their capability as reverse osmosis membranes is lost once the membranes are dried. These membranes have also exhibited deficiencies such as alkali or acidic degradation and biological degradation resulting in short life. Furthermore, these membranes are not used widely in a separation or recovery of valuable materials from a liquid mixture containing organic chemicals, because the membranes have low selectivity for valuable organic materials.
Other types of semipermeable membranes which are also in use include membranes fabricated from polyamides (please refer to U.S. Pat. No. 3,567,632, for example), polyimides, polysulfoneamides and other polymeric organic materials.
In later developments the processes have been utilized for preparing an ultra-thin film or skin separately from a porous supporting layer. Membranes so prepared have become known as composite membranes. In preparing such membranes it is possible to tailor both the ultra-thin film and the porous supporting layer in such a manner that each component possesses the most desirable properties. A process for preparation of composite membranes is described in U.S. Pat. Nos. 3,744,642, 3,926,798, and in P. B. Reports Nos. 234,198 and 248,670. These prior art membranes have, however, been generally subject to deficiencies such as compaction, resulting in short life, as well as undesirably low flux or solute rejection, all resulting in inefficient operation.